Variable displacement engine control system and method

ABSTRACT

Methods and systems are provided for improving the performance of a variable displacement engine. Split injection and spark retard may be used in active cylinder during a VDE mode to heat an exhaust catalyst and extend the duration of VDE mode operation. Split injection and spark retard may also be used in reactivated cylinders at a time of cylinder reactivation to improve restart combustion stability.

FIELD

The present application relates to adjusting fuel injection strategywhen operating in or transitioning between modes in a variabledisplacement internal combustion engine (VDE).

BACKGROUND AND SUMMARY

Engines may be configured to operate with a variable number of active ordeactivated cylinders to increase fuel economy, while optionallymaintaining the overall exhaust mixture air-fuel ratio aboutstoichiometry. Such engines are known as variable displacement engines(VDE). In some examples, a portion of an engine's cylinders may bedisabled during selected conditions, where the selected conditions canbe defined by parameters such as a speed/load window, as well as variousother operating conditions including vehicle speed. A VDE control systemmay disable selected cylinders through the control of a plurality ofcylinder valve deactivators that affect the operation of the cylinder'sintake and exhaust valves, or through the control of a plurality ofselectively deactivatable fuel injectors that affect cylinder fueling.When transitioning between a VDE mode (where one or more cylinders aredeactivated) and a non-VDE mode (where all the cylinders are active),the control system may adjust one or more engine operating parameters toreduce disturbances (e.g., torque disturbances) and attenuate thedisturbance during the transition.

One example approach for engine control during a VDE transition is shownby Pallett et al in U.S. Pat. No. 7,225,782. Therein, the VDE engine iscoupled in a hybrid electric vehicle having an electric motor. Whenenabling or disabling a cylinder, torque from the motor is varied tocompensate for transient changes in engine output torque caused by theenabling or disabling of the cylinder. In particular, the electric motoris controlled to mask any drivability issues associated with VDEtransitions and/or poor combustion stability.

However the inventors herein have identified potential issues with suchan approach. As one example, combustion stability may be degraded duringthe transition. Specifically, when transitioning from the VDE mode (orpartial cylinder mode) to the non-VDE mode (or full cylinder mode),cylinder load decreases based on the decrease in aircharge. The lightercylinder loads generally have less stable combustion and the interactionwith the transient fuel compensation, and other cylinder conditions thatare different than the operating cylinders due to cooling duringdeactivation, may contribute to less stable combustion duringreactivation. If the engine is equipped for exhaust gas recirculation,EGR control used during the transition may exacerbate the combustionissues. In particular, the EGR may continue to interfere with thelighter cylinder load until the EGR delivered to the cylinders has beensufficiently bled down to reduce combustion issues. In some embodiments,charge motion control valves (CMCVs) may be used to adjust thein-cylinder motion of the air-fuel mixture delivered to the cylinderduring the transition. High in-cylinder motion results in better mixing,and more stable combustion. However, due to the slow response time ofthe CMCV (e.g., the CMCV not shutting quickly enough when transitioningto the lower cylinder load), combustion stability may be compromised.The poor combustion conditions can also lead to slow burns or evenmisfires. Overall, combustion stability and engine performance may bedegraded.

In one example, some of the above issues may be at least partlyaddressed by a method for an engine comprising: selectively deactivatingone or more engine cylinders responsive to operating conditions, andduring reactivation of the cylinders, operating the reactivatedcylinders with split fuel injection. Specifically, the fuel injection ofthe reactivated cylinders may be transiently shifted to each of anintake stroke and a compression stroke injection for a number ofcombustion events. In this way, restart combustion stability is improvedand torque disturbances during a transition out of a VDE mode ofoperation are reduced.

In one example, a variable displacement engine may be configured withselectively deactivatable fuel injectors. In response to selecteddeactivation conditions, such as reduced engine load or torque demand,one or more cylinders may be deactivated and the engine may be operatedin a VDE mode. For example, the engine may be operated with half thecylinders deactivated and with the remaining active cylinders operatingat a higher cylinder load. During the deactivation, the active cylindersmay be operated with fuel delivered as a single intake stroke injection.In addition, due to the higher average cylinder load, the cylinders maybe operable with EGR without incurring combustion stability issuesotherwise incurred at low engine loads. The use of EGR during VDEoperation provides additional fuel economy benefits.

In response to reactivation conditions, such as increased engine load ortorque demand, the deactivated cylinders may be reactivated and theengine may resume a non-VDE mode of operation wherein all the cylindersare operated at a lower average cylinder load. In addition, EGR may bestopped (e.g., by closing an EGR valve) due to the engine's reduced EGRtolerance during the reactivation when the cylinders are transitioningto a lower cylinder load. Even though the EGR valve is closed, due totransport delays along the EGR passage, EGR may purge from the airintake system slower than desired, resulting in increased EGR dilutionof intake air during the reactivation. Also, during engine operation inthe VDE mode, the exhaust catalyst may becomes saturated with oxygen andmay need to be regenerated during cylinder reactivation. Therefore, toreduce combustion stability issues arising from the increased EGRdilution of intake air, as well as to expedite catalyst regeneration,during the reactivation, for a number of combustion events since thereactivation, the reactivated cylinders may be operated with fueldelivered as a split fuel injection. For example, fuel may be deliveredas at least a first intake stroke injection and a second compressionstroke injection. In addition, spark timing may be retarded. A splitratio of fuel delivered in the first intake stroke injection relative tofuel delivered in the second compression stroke injection may beadjusted based on one or more of a duration of the deactivation, atemperature or oxygen loading of an exhaust catalyst coupled downstreamof the reactivated cylinders, an amount of spark retard applied, and EGRlevel at reactivation. By temporarily shifting to a split fuel injectionwhile retarding spark, reactivation of the exhaust catalyst can beexpedited, improving exhaust emissions. In addition, combustionstability issues arising from the decrease in individual cylinder loadduring the transition out of the VDE mode can be better addressed,particularly in the presence of the EGR. The split ratio may also beadjusted based on other engine operating parameters, such as the alcoholcontent of the injected fuel, to compensate for drivability and enginestumble issues. As such, the use of a split injection and spark retardmay be continued for a number of combustion events until the enginespeed is at or above a threshold speed where combustion stability isimproved (e.g., at or above idling speed) and/or until the EGR issufficiently bled down.

In this way, by operating reactivated cylinders with split fuelinjection for a number of combustion events during a reactivation fromVDE mode of engine operation, restart combustion stability of thecylinders is improved. By injecting at least a portion of the fuelduring an intake stroke and a remaining portion in a compression stroke,exhaust catalyst regeneration following the VDE mode of operation can beexpedited, providing emissions benefits. By further adjusting the splitratio based on an alcohol content of the injected fuel, poor combustionevents that may result in a stumble may be reduced. As such, thisreduces drivability deterioration from mixed fuel usage. In addition,the use of a split injection during reactivation improves the EGR usagein the active cylinders during the preceding deactivation. Overall,engine performance is improved.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-2 show example embodiments of an engine and exhaust systemlayout.

FIG. 3 shows a partial engine view.

FIG. 4 shows a high level flow chart for adjusting fuel injection duringa transition between VDE and non-VDE modes of engine operation.

FIG. 5 shows a high level flow chart for adjusting fuel injection duringa VDE mode of engine operation responsive to a drop in exhaust catalysttemperature.

FIG. 6 shows an example fuel injection adjustment during a transitionfrom VDE to non-VDE mode of operation.

FIGS. 7-9 show example fuel injection adjustments during a VDE mode ofoperation or during a transition from VDE to non-VDE mode of operationthat may be used to manage an exhaust catalyst temperature andcombustion stability.

DETAILED DESCRIPTION

Methods and systems are provided for adjusting a fuel injection profilewhen operating a variable displacement engine, such as the engine ofFIGS. 1-3. The fuel injection profile may be adjusted for activecylinders during a VDE mode of operation to expedite exhaust catalystheating and thereby prolong operation in the VDE mode. Alternatively,the fuel injection profile may be adjusted for reactivated cylindersduring a transition from VDE mode to non-VDE mode of operation to reducetorque disturbances and combustion stability issues while improvingexhaust emissions. A controller may be configured to perform a routine,such as the routine of FIG. 4, to shift a fuel injection profile forselected cylinder from a single intake stroke injection to at least afirst intake stroke injection and a second compression stroke injection.For example, as shown at FIG. 5 and FIG. 7, the controller may shift theactive cylinders during a VDE mode to split fuel injection. As anotherexample, as shown at FIG. 6 and FIGS. 8-9, the controller may shiftcylinders being reactivated during a transition out of VDE mode to splitfuel injection. In this way, combustion stability issues can be improvedwhen transitioning out of a VDE mode of operation, and cylinderreactivation can be expedited.

FIGS. 1-2 show example embodiments 100 and 200 of engine 10 wherein theengine is configured as a variable displacement engine (VDE). Variabledisplacement engine 10 includes a plurality of combustion chambers orcylinders 31. The plurality of cylinders 31 of engine 10 are arranged asgroups of cylinders on distinct engine banks. In the depicted example,engine 10 includes two engine banks 14A, 14B. Thus, the cylinders arearranged as a first group of cylinders (four cylinders in the depictedexample) arranged on first engine bank 14A and a second group ofcylinders (four cylinders in the depicted example) arranged on secondengine bank 14B. It will be appreciated that while the embodimentsdepicted in FIGS. 1-2 show a V-engine with cylinders arranged ondifferent banks, this is not meant to be limiting, and in alternateembodiments, the engine may be an in-line engine with all enginecylinders on a common engine bank.

Variable displacement engine 10 can receive intake air via an intakepassage 142 communicating with branched intake manifold 44A, 44B.Specifically, first engine bank 14A receives intake air from intakepassage 142 via first intake manifold 44A while second engine bank 14Breceives intake air from intake passage 142 via second intake manifold44B. While engine banks 14A, 14B are shown with distinct intakemanifolds, it will be appreciated that in alternate embodiments, theymay share a common intake manifold or a portion of a common intakemanifold. The amount of air supplied to the cylinders of the engine canbe controlled by adjusting a position of throttle 62. Additionally, anamount of air supplied to each group of cylinders on the specific bankscan be adjusted by varying an intake valve timing of one or more intakevalves coupled to the cylinders.

With reference to FIG. 1, combustion products generated at the cylindersof first engine bank 14A are directed to one or more exhaust catalystsin first exhaust manifold 48A where the combustion products are treatedbefore being vented to the atmosphere. A first emission control device70A is coupled to first exhaust manifold 48A. First emission controldevice 70A may include one or more exhaust catalysts, such as aclose-coupled catalyst. In one example, the close-coupled catalyst atemission control device 70A may be a three-way catalyst. Exhaust gasgenerated at first engine bank 14A is treated at emission control device70A before being directed to first underbody emission control device80A. First underbody emission control device 80A may include a firstunderbody exhaust catalyst 82A and a second underbody exhaust catalyst84A. In particular, the first underbody 82A and the second underbodycatalyst 84A may be integrated in the underbody emission control device80A in face-sharing contact with each other. In one example, firstunderbody exhaust catalyst 82A includes an SCR catalyst configured forselective catalytic reduction wherein NOx species are reduced tonitrogen using ammonia. As another example, second underbody exhaustcatalyst 84A includes a three-way catalyst. First underbody exhaustcatalyst 82A is positioned upstream of the second underbody exhaustcatalyst 84A (in a direction of exhaust flow) in the underbody emissioncontrol device 80A but downstream of a third close-coupled exhaustcatalyst (included in emission control device 70A).

Exhaust that is treated upon passage through first emission controldevice 70A and first underbody emission control device 80A is thendirected towards exhaust junction 55 along first exhaust manifold 48A.From there, the exhaust can be directed to the atmosphere via commonexhaust passage 50.

Combustion products generated at the cylinders of second engine bank 14Bare exhausted to the atmosphere via second exhaust manifold 48B. Asecond emission control device 70B is coupled to second exhaust manifold48B. Second emission control device 70B may include one or more exhaustcatalysts, such as a close-coupled catalyst. In one example, theclose-coupled catalyst at emission control device 70A may be a three-waycatalyst. Exhaust gas generated at second engine bank 14B is treated atemission control device 70B before being directed to second underbodyemission control device 80B. Second underbody emission control device80B may also include a first underbody exhaust catalyst 82B and a secondunderbody exhaust catalyst 84B. In particular, the first underbodycatalyst 82B and the second underbody catalyst 84B may be integrated inthe underbody emission control device 80B in face-sharing contact witheach other. In one example, first underbody exhaust catalyst 82Bincludes an SCR catalyst while second underbody exhaust catalyst 84Bincludes a three-way catalyst. Second underbody exhaust catalyst 82B ispositioned upstream of the second underbody exhaust catalyst 84B (in adirection of exhaust flow) in the underbody emission control device 80Bbut downstream of a third close-coupled exhaust catalyst (included inemission control device 70B).

While the embodiment of FIG. 1 shows each engine bank coupled torespective underbody emission control devices, in alternate embodiments,such as shown at FIG. 2, each engine bank is coupled to respectiveemission control devices 70A, 70B but to a common underbody emissioncontrol device 80. In embodiment 200 depicted at FIG. 2, the commonunderbody emission control device 80 is positioned downstream of exhaustjunction 55 and common exhaust passage 50. Common underbody emissioncontrol device 80 is shown with first underbody exhaust catalyst 82positioned upstream of and integratably coupled to second underbodyexhaust catalyst 84 (in a direction of exhaust flow) in the underbodyemission control device 80.

Various air-to-fuel ratio sensors may be coupled to engine 10. Forexample, a first air-to-fuel ratio sensor 72 may be coupled to the firstexhaust manifold 48A of first engine bank 14A, downstream of firstemission control device 70A while a second air-to-fuel ratio sensor 74is coupled to the second exhaust manifold 48B of second engine bank 14B,downstream of second emission control device 70B. In furtherembodiments, additional air-to-fuel ratio sensors may be coupledupstream of the emission control devices. Still other air-to-fuel ratiosensors may be included, for example, coupled to the underbody emissioncontrol device(s). As elaborated at FIG. 3, the air-to-fuel ratiosensors may include oxygen sensors, such as EGO, HEGO, or UEGO sensors.In one example, the downstream air-to-fuel ratio sensors 72, 74 coupleddownstream of emission control devices 70A, 70B may be HEGO sensors usedfor catalyst monitoring while the upstream air-to-fuel ratio sensorscoupled upstream of emission control devices 70A, 70B (when included)are UEGO sensors used for engine control.

Further still, one or more temperature sensors may be coupled to theemission control device for estimating a temperature of exhaust enteringthe device and for estimating a temperature of the emission controldevice. As elaborated herein, a controller may adjust fuel injection toone or more engine cylinders based on the estimated temperature. Forexample, as elaborated at FIGS. 4-6, the controller may adjust fuelinjection of engine cylinders on a deactivated during reactivation basedon the estimated temperature. Alternatively, the controller may adjustfuel injection of active engine cylinders during engine operation in aVDE mode based on the estimated temperature.

One or more engine cylinders may be selectively deactivated duringselected engine operating conditions. For example, during low engineloads, one or more cylinders of a selected engine bank may beselectively deactivated. Even though the engine load is lower, bydeactivating selected cylinders, the average cylinder load of theremaining active cylinders is increased, improving pumping efficiency.In addition, higher EGR usage may be possible in the active cylinders,even though the engine load is lower. Specifically, EGR may be used whenengine loads are higher than a threshold to provide fuel economy andemissions benefits. However, at lower engine loads, EGR usage may not bepreferred due to combustion stability issues. By operating the activecylinders at a higher average load, their EGR tolerance is improved anda higher EGR rate can be used during a VDE mode of operation even atoverall lower engine loads. The synergistic use of EGR and VDE furtherimproves engine fuel economy.

The selective cylinder deactivation may include deactivating fuel andspark on the selected engine cylinders (or a selected engine bank if anentire bank is deactivated, such as in flat crankshaft arrangements). Inaddition, an intake and/or exhaust valve timing may be adjusted so thatsubstantially no air is pumped through the inactive engine bank whileair continues to flow through the active engine bank. In someembodiments, the deactivated cylinders may have cylinder valves heldclosed during one or more engine cycles, wherein the cylinder valves aredeactivated via hydraulically actuated lifters, or via a cam profileswitching (CPS) mechanism in which a cam lobe with no lift is used fordeactivated valves. In one example, an engine controller may selectivelydeactivate all the cylinders of a given engine bank (either 14A or 14B)during shift to a VDE mode and then reactivate the cylinders during ashift back to a non-VDE mode.

By selectively deactivating engine cylinders during low engine loadconditions, engine pumping losses and friction losses are reduced, andfuel economy is improved. However, the continued air flow through theinactive bank can lead to a drop in temperature at emission controldevices positioned downstream of the inactive bank. In particular, inflat engine crankshaft arrangements having even firing, such as a V6 ora V10 engine, where an entire bank of cylinders is deactivated duringthe VDE mode, or engine with NVH treatments that allow deactivation ofan entire bank, an exhaust catalyst coupled downstream of the inactivebank may need to be reactivated due to catalyst cooling when the bank ofcylinders is not operated (and oxygen saturation if the valves continueto pump air through the inactive cylinders).

It will be appreciated that in other engine crankshaft arrangements,such as a V8 engine, during a VDE mode, each bank may have a set ofcylinders deactivated. For example, the outer cylinders and inner ofeach bank may be alternately deactivated. In these arrangements where anentire bank of cylinders is not deactivated during the VDE mode, theexhaust catalyst coupled downstream of the emission control device maynot incur a temperature drop.

If the duration of VDE operation is short, significant enrichment and orspark retard may be required after exiting the VDE mode to quicklyreactivate the exhaust catalyst. This enrichment adds a fuel penalty. Insome cases, the fuel penalty associated with the reactivation cannullify or even exceed the fuel economy benefit of the VDE mode ofengine operation.

As elaborated herein with reference to FIGS. 3-6, a controller mayoperate the active cylinders with a split fuel injection for a durationto maintain exhaust catalyst temperatures above a threshold, therebydelaying the need for significant enrichment during the reactivation. Inaddition, the use of a split fuel injection may also allow operation inthe VDE mode to be prolonged, increasing the fuel economy benefits ofthe VDE mode of operation. The controller may also transition fuelinjection of one or more reactivated engine cylinders duringreactivation to a split fuel injection to improve restart combustionstability and reduce torque disturbances and combustion instabilityduring the reactivation.

In embodiments where the engine was operating with EGR during the VDEmode of operation, the use of split injection during the reactivationmay be adjusted based on the EGR and maintained while the EGR is bleddown. By using a split injection at the time of reactivation, a higherEGR rate can be used in VDE mode because the EGR can be bled down fromthis higher rate and the transition to the lower cylinder load can beadvanced without degrading combustion. Alternatively, the transition tocylinder reactivation can be performed at a higher EGR level during theEGR bleed down period than would have been possible without the use of asplit injection, reducing the delay time before transition to cylinderreactivation.

Specifically, when the cylinders are reactivated, EGR may be terminated,and the cylinders may resume operation with a higher average cylinderload (even though the engine load is higher). Since EGR purges out ofthe intake system slower than required, due to the long transport delayincurred in the EGR passage, the high dilution of air with EGR at thelow cylinder load can increase combustion instability and a propensityfor misfires. By using split fuel injection during the reactivationwhile EGR is purged from the intake system, combustion stability issuesat low cylinders loads, in particular those due to elevated dilution,can be better addressed. Example fuel injection adjustments are depictedat FIGS. 7-8.

It will be appreciated that in some embodiments, the engineconfiguration of FIG. 2 may employ a different catalyst reactivationstrategy compared to the strategy used for the engine configuration ofFIG. 1. This is because the underbody emission control devices of FIG. 2may convert the emissions, if either device 70 a or 70 b is inactive, tocombusted gas.

FIG. 3 depicts an example embodiment 300 of a combustion chamber orcylinder of internal combustion engine 10. Engine 10 may receive controlparameters from a control system including controller 12 and input froma vehicle operator 130 via an input device 132. In this example, inputdevice 132 includes an accelerator pedal and a pedal position sensor 134for generating a proportional pedal position signal PP. Cylinder (hereinalso “combustion chamber’) 14 of engine 10 may include combustionchamber walls 136 with piston 138 positioned therein. Piston 138 may becoupled to crankshaft 140 so that reciprocating motion of the piston istranslated into rotational motion of the crankshaft. Crankshaft 140 maybe coupled to at least one drive wheel of the passenger vehicle via atransmission system. Further, a starter motor may be coupled tocrankshaft 140 via a flywheel to enable a starting operation of engine10.

Cylinder 14 can receive intake air via a series of intake air passages142, 144, and 146. Intake air passage 146 may communicate with othercylinders of engine 10 in addition to cylinder 14. In some embodiments,one or more of the intake passages may include a boosting device such asa turbocharger or a supercharger. For example, FIG. 2 shows engine 10configured with a turbocharger including a compressor 174 arrangedbetween intake passages 142 and 144, and an exhaust turbine 176 arrangedalong exhaust passage 148. Compressor 174 may be at least partiallypowered by exhaust turbine 176 via a shaft 180 where the boosting deviceis configured as a turbocharger. However, in other examples, such aswhere engine 10 is provided with a supercharger, exhaust turbine 176 maybe optionally omitted, where compressor 174 may be powered by mechanicalinput from a motor or the engine. A throttle 20 including a throttleplate 164 may be provided along an intake passage of the engine forvarying the flow rate and/or pressure of intake air provided to theengine cylinders. For example, throttle 20 may be disposed downstream ofcompressor 174 as shown in FIG. 1, or alternatively may be providedupstream of compressor 174.

Exhaust passage 148 may receive exhaust gases from other cylinders ofengine 10 in addition to cylinder 14. Exhaust gas sensor 128 is showncoupled to exhaust passage 148 upstream of emission control device 178.Sensor 128 may be selected from among various suitable sensors forproviding an indication of exhaust gas air/fuel ratio such as a linearoxygen sensor or UEGO (universal or wide-range exhaust gas oxygen), atwo-state oxygen sensor or EGO (as depicted), a HEGO (heated EGO), aNOx, HC, or CO sensor, for example. Emission control device 178 may be athree way catalyst (TWC), NOx trap, various other emission controldevices, or combinations thereof.

Exhaust temperature may be measured by one or more temperature sensors(not shown) located in exhaust passage 148. Alternatively, exhausttemperature may be inferred based on engine operating conditions such asspeed, load, air-fuel ratio (AFR), spark retard, etc. Further, exhausttemperature may be computed by one or more exhaust gas sensors 128. Itmay be appreciated that the exhaust gas temperature may alternatively beestimated by any combination of temperature estimation methods listedherein.

Each cylinder of engine 10 may include one or more intake valves and oneor more exhaust valves. For example, cylinder 14 is shown including atleast one intake poppet valve 150 and at least one exhaust poppet valve156 located at an upper region of cylinder 14. In some embodiments, eachcylinder of engine 10, including cylinder 14, may include at least twointake poppet valves and at least two exhaust poppet valves located atan upper region of the cylinder.

Intake valve 150 may be controlled by controller 12 by cam actuation viacam actuation system 151. Similarly, exhaust valve 156 may be controlledby controller 12 via cam actuation system 153. Cam actuation systems 151and 153 may each include one or more cams and may utilize one or more ofcam profile switching (CPS), variable cam timing (VCT), variable valvetiming (VVT) and/or variable valve lift (VVL) systems that may beoperated by controller 12 to vary valve operation. The operation ofintake valve 150 and exhaust valve 156 may be determined by valveposition sensors (not shown) and/or camshaft position sensors 155 and157, respectively. In alternative embodiments, the intake and/or exhaustvalve may be controlled by electric valve actuation. For example,cylinder 14 may alternatively include an intake valve controlled viaelectric valve actuation and an exhaust valve controlled via camactuation including CPS and/or VCT systems. In still other embodiments,the intake and exhaust valves may be controlled by a common valveactuator or actuation system, or a variable valve timing actuator oractuation system.

Cylinder 14 can have a compression ratio, which is the ratio of volumeswhen piston 138 is at bottom center to top center. Conventionally, thecompression ratio is in the range of 9:1 to 10:1. However, in someexamples where different fuels are used, the compression ratio may beincreased. This may happen, for example, when higher octane fuels orfuels with higher latent enthalpy of vaporization are used. Thecompression ratio may also be increased if direct injection is used dueto its effect on engine knock.

In some embodiments, each cylinder of engine 10 may include a spark plug192 for initiating combustion. Ignition system 190 can provide anignition spark to combustion chamber 14 via spark plug 192 in responseto spark advance signal SA from controller 12, under select operatingmodes.

In some embodiments, each cylinder of engine 10 may be configured withone or more injectors for delivering fuel to the cylinder. As anon-limiting example, cylinder 14 is shown including two fuel injectors166 and 170. Fuel injectors 166 and 170 may be configured to deliverfuel received from fuel system 8 via a high pressure fuel pump, and afuel rail. Alternatively, fuel may be delivered by a single stage fuelpump at lower pressure, in which case the timing of the direct fuelinjection may be more limited during the compression stroke than if ahigh pressure fuel system is used. Further, the fuel tank may have apressure transducer providing a signal to controller 12.

Fuel injector 166 is shown coupled directly to cylinder 14 for injectingfuel directly therein in proportion to the pulse width of signal FPW-1received from controller 12 via electronic driver 168. In this manner,fuel injector 166 provides what is known as direct injection (hereafterreferred to as “DI”) of fuel into combustion cylinder 14. While FIG. 1shows injector 166 positioned to one side of cylinder 14, it mayalternatively be located overhead of the piston, such as near theposition of spark plug 192. Such a position may improve mixing andcombustion when operating the engine with an alcohol-based fuel due tothe lower volatility of some alcohol-based fuels. Alternatively, theinjector may be located overhead and near the intake valve to improvemixing.

Fuel injector 170 is shown arranged in intake passage 146, rather thanin cylinder 14, in a configuration that provides what is known as portinjection of fuel (hereafter referred to as “PFI”) into the intake portupstream of cylinder 14. Fuel injector 170 may inject fuel, receivedfrom fuel system 8, in proportion to the pulse width of signal FPW-2received from controller 12 via electronic driver 171. Note that asingle driver 168 or 171 may be used for both fuel injection systems, ormultiple drivers, for example driver 168 for fuel injector 166 anddriver 171 for fuel injector 170, may be used, as depicted.

Fuel injectors 166 and 170 may have different characteristics. Theseinclude differences in size, for example, one injector may have a largerinjection hole than the other. Other differences include, but are notlimited to, different spray angles, different operating temperatures,different targeting, different injection timing, different spraycharacteristics, different locations etc. Moreover, depending on thedistribution ratio of injected fuel among injectors 166 and 170,different effects may be achieved.

Fuel may be delivered by both injectors to the cylinder during a singlecycle of the cylinder. For example, each injector may deliver a portionof a total fuel injection that is combusted in cylinder 14. As such,even for a single combustion event, injected fuel may be injected atdifferent timings from the port and direct injector. Furthermore, for asingle combustion event, multiple injections of the delivered fuel maybe performed per cycle. The multiple injections may be performed duringthe compression stroke, intake stroke, or any appropriate combinationthereof.

As described above, FIG. 2 shows only one cylinder of a multi-cylinderengine. As such each cylinder may similarly include its own set ofintake/exhaust valves, fuel injector(s), spark plug, etc. It will beappreciated that engine 10 may include any suitable number of cylinders,including 2, 3, 4, 5, 6, 8, 10, 12, or more cylinders. Further, each ofthese cylinders can include some or all of the various componentsdescribed and depicted by FIG. 2 with reference to cylinder 14.

The engine may further include one or more exhaust gas recirculationpassages for recirculating a portion of exhaust gas from the engineexhaust to the engine intake. As such, by recirculating some exhaustgas, an engine dilution may be affected which may improve engineperformance by reducing engine knock, peak cylinder combustiontemperatures and pressures, throttling losses, and NOx emissions. In thedepicted embodiment, exhaust gas may be recirculated from exhaustpassage 148 to intake passage 144 via EGR passage 141. The amount of EGRprovided to intake passage 144 may be varied by controller 12 via EGRvalve 143. Further, an EGR sensor 145 may be arranged within the EGRpassage and may provide an indication of one or more pressure,temperature, and concentration of the exhaust gas.

Controller 12 is shown in FIG. 1 as a microcomputer, includingmicroprocessor unit 106, input/output ports 108, an electronic storagemedium for executable programs and calibration values shown as read onlymemory chip 110 in this particular example, random access memory 112,keep alive memory 114, and a data bus. Controller 12 may receive varioussignals from sensors coupled to engine 10, in addition to those signalspreviously discussed, including measurement of inducted mass air flow(MAF) from mass air flow sensor 122; engine coolant temperature (ECT)from temperature sensor 116 coupled to cooling sleeve 118; a profileignition pickup signal (PIP) from Hall effect sensor 120 (or other type)coupled to crankshaft 140; throttle position (TP) from a throttleposition sensor; and manifold absolute pressure signal (MAP) from sensor124. Engine speed signal, RPM, may be generated by controller 12 fromsignal PIP. Manifold pressure signal MAP from a manifold pressure sensormay be used to provide an indication of vacuum, or pressure, in theintake manifold. Still other sensors may include fuel level sensors andfuel composition sensors coupled to the fuel tank(s) of the fuel system.

Storage medium read-only memory 110 can be programmed with computerreadable data representing instructions executable by processor 106 forperforming the methods described below as well as other variants thatare anticipated but not specifically listed. Example methods arediscussed with reference to FIGS. 4-6.

Now turning to FIG. 4, an example routine 400 is shown for adjustingfuel injection in one or more engine cylinders of a VDE engine. The fuelinjection may be adjusted during a transition from VDE to non-VDE modeto reduce torque disturbances during the transition and improve restartcombustion stability. The fuel injection may also be adjusted duringoperation in the VDE mode to rapidly heat an exhaust catalyst andprolong operation in the VDE mode.

At 402, the routine includes confirming an engine cold-start condition.In one example, an engine cold-start may be confirmed if an enginecoolant temperature is below a threshold, an exhaust catalysttemperature is below a light-off temperature, an ambient temperature isbelow a threshold, and/or the engine has been shut down for more than athreshold duration. If an engine cold start is not confirmed (that is anengine hot start is confirmed), then at 404, the routine includesoperating the engine with a single fuel injection. The single fuelinjection (amount, timing, duration, etc.) may be based on engineoperating conditions. For example, fuel may be delivered as a singleintake stroke injection. Further, based on the engine operatingconditions, the single intake stroke injection may be delivered viadirect injection or port injection.

If an engine cold-start is confirmed, then at 406, the routine includesoperating the engine with a split fuel injection that is based on enginetemperature. Specifically, for a duration of the cold start, a splitfuel injection may be performed with at least some fuel delivered as anintake stroke injection and a compression stroke injection. Optionally,at least some fuel may be delivered as an exhaust stroke injection. Inone example, the fuel delivered in the exhaust stroke may be providedvia port injection while the fuel delivered in the intake andcompression strokes may be delivered via direct injection. In additionspark timing may be retarded. By using a split injection during the coldstart wherein at least some fuel is direct injected during thecompression stroke and the remaining part of the fuel during the intakestroke, a catalyst light-off temperature can be attained without raisingexhaust particulate matter (PM) emissions and degrading enginecombustion stability. In one example, during an engine cold start, anintake stroke injection may be performed at 240 deg BTDC, a compressionstroke injection may be performed at 40 deg BTDC, and a split ratio of60/40 may be applied. In addition, spark timing may be retarded to 15deg ATDC.

After a cold start at 406, or a hot start at 404, the routine proceedsto 408 wherein engine operating conditions are estimated and/or measuredand it is determined if VDE conditions have been met. The estimatedoperating conditions may include, for example, engine speed, desiredtorque (for example, from a pedal-position sensor), manifold pressure(MAP), manifold air flow (MAF), BP, engine temperature, catalysttemperature, intake temperature, spark timing, air temperature, knocklimits, etc. In one example, if engine load is below a threshold, VDEconditions may be considered met. If VDE conditions are not met, at 410,all the engine cylinders may be maintained active and the engine may beoperated in a non-VDE mode.

If VDE conditions are considered met, then at 412, the routine includesselectively deactivating one or more engine cylinders responsive to theoperating conditions. For example, one or more engine cylinders of afirst engine bank (or a first group of cylinders) may be deactivatedwhile a second engine bank (or second group of cylinders) remainsactive. The cylinders may be deactivated via selectively deactivatablefuel injectors. In addition to fuel, spark may also be deactivated fromthe cylinders. As an example, in flat engine crankshaft arrangementshaving even firing, such as a V6 or a V10 engine, an entire bank ofcylinders may be deactivated during the VDE mode. In alternate otherengine crankshaft arrangements, such as a V8 engine, the outermost twocylinders and inner two of each bank may be alternately deactivated.

At 413, based on engine operating conditions and the average cylinderload of the active cylinders, an amount of engine dilution that can beprovided may be determined. Based on the dilution requirement, an EGRvalve may be adjusted to provide the desired dilution. As such, duringthe VDE mode of operation, due to the higher average cylinder load, ahigher EGR rate can be used (as compared to the same engine load beingprovided without a VDE mode of operation). This allows EGR benefits tobe achieved even during low engine load conditions.

At 414 the routine includes, during the deactivation, monitoring atemperature of an emission control device coupled downstream of the oneor more engine cylinders. The monitoring may include, for example,monitoring a temperature of an emission control device coupleddownstream of the first (deactivated) engine bank but not the second(active) engine bank. Alternatively, the monitoring may includemonitoring a temperature of an emission control device coupleddownstream of each of the first and second engine banks. The temperaturemay be estimated by a temperature sensor or inferred based on operatingconditions. In another example, the monitoring may be performed by anexhaust UEGO sensor. It will be appreciated that in engine systemshaving flat crankshaft arrangements with even firing, such as a V6 or aV10 engine, where an entire bank of cylinders is deactivated during theVDE mode, the temperature drop at the exhaust catalyst coupled to theinactive bank may be more pronounced (e.g., a larger drop intemperature) than in engine systems where an entire bank is notdeactivated (such as in a V8 engine wherein either the outer cylindersor inner cylinders of each bank are deactivated).

At 416, it may be determined if the monitored temperature is lower thana first threshold. If the temperature is above the first threshold,sufficient emission control device heating may be inferred and at 418,it may be determined if non-VDE conditions have been met. In oneexample, non-VDE conditions may be considered met if the engine load ortorque demand is higher than a threshold. If non-VDE conditions are notmet, then at 428, the engine may continue operation in the VDE mode withsome of the cylinders deactivated. If non-VDE conditions are met, thenat 424, the routine includes reactivating the previously deactivatedengine cylinders. This may include resuming fuel injection and spark inthe cylinders. As such, by reactivating the cylinders, an average loadof each cylinder is reduced, as compared to the average load of eachcylinder during the VDE mode, even though the engine load may be higher.

At 426, during reactivation of the cylinders, the controller mayoptionally operate the reactivated cylinders with split fuel injection.Additionally, spark timing may be retarded during the reactivation tofurther expedite heating and reactivation of the exhaust catalyst. Aselaborated at FIG. 6, this includes operating the reactivated cylinderswith split fuel injection for a number of combustion events followingthe reactivation and then resuming single fuel injection. The use of asplit fuel injection during the reactivation improves the combustionstability of the reactivated cylinders which are now operating at lowindividual cylinder loads. The split fuel injection may be continued atleast until the individual cylinder load increases, such as when theengine speed is at or above a threshold speed (e.g. idling speed).Additionally, if the engine was operating with EGR during the VDE mode,the EGR may be ramped down during the reactivation, and the split fuelinjection may be maintained during the reactivation until the EGR hasbeen ramped down to a desired level. The use of a split injection untilthe EGR has been sufficiently purged improves the cylinders' combustionstability at the low load high dilution conditions.

The split fuel injection may include at least a first intake strokeinjection and a second compression stroke injection. In addition to asplit fuel injection, the cylinders may be operated with spark retard tomaximize exhaust heat generation. The details of the split fuelinjection (timing, split ratio, pressure, amount, etc.) may be adjustedbased on various parameters including, for example, a duration of thepreceding deactivation (that is, duration in VDE mode), exhaust catalysttemperature, engine speed-load conditions at time of reactivation, etc.,so as to improve restart combustion stability. In addition, torquedisturbances during the transition out of VDE mode and into non-VDE modecan be reduced using the split injection. As such, the split fuelinjection used during the reactivation may differ from the split fuelinjection used during the engine cold-start. For example, the splitratio used during the reactivation may include relatively less fueldelivered in the compression stroke and relatively more fuel deliveredin the intake stroke. In addition, the timing of the compression strokeinjection during the reactivation may be closer to intake BDC while theinjection during the cold start is closed to compression TDC. In oneexample, during the transition out of the VDE mode, the split fuelinjection in the reactivated cylinders may include an intake strokeinjection performed at 240 deg BTDC, a compression stroke injectionperformed at 40 deg BTDC, and a split ratio of 60/40(intake:compression) may be applied. In addition, spark timing may beretarded to 15 deg ATDC. In another example, during the transition outof the VDE mode, the split fuel injection in all the reactivatedcylinders may include an intake stroke injection performed at 220 degBTDC, a compression stroke injection performed at 35 deg BTDC, and asplit ratio of 70/30 (intake:compression) may be applied. In addition,spark timing may be retarded to 14 deg ATDC.

Returning to 416, if the monitored temperature is below the firstthreshold (Thr_1), then at 420, it may be determined if the monitoredtemperature has fallen below a second threshold (Thr_2) lower than thefirst threshold. The first threshold may be based on, for example, acatalyst light-off temperature. The second threshold may be based on thefirst threshold and/or the catalyst light-off temperature. In alternateembodiments, a drop rate of the temperature may be determined. Inresponse to the temperature falling below the first threshold, butremaining above the second threshold (or a slower drop in temperature),at 422, the routine includes operating the active cylinders of theengine with split fuel injection. As elaborated with reference to FIG.5, the operating may be performed for a number of combustion eventsfollowing the falling of the temperature below the threshold after whichcylinder fuel injection based on engine speed-load conditions may beresumed. For example, the active cylinders may resume single fuelinjection. The split fuel injection may include at least a first intakestroke injection and a second compression stroke injection. The detailsof the split fuel injection (timing, split ratio, pressure, amount,etc.) may be adjusted based on the monitored emission control device (orexhaust catalyst) temperature to expedite heating of the catalyst. Indoing so, engine operation in the VDE mode may be prolonged. In additionto a split fuel injection, the cylinders may be operated with sparkretard to maximize exhaust heat generation. In one example, during theVDE mode, the split fuel injection in the active cylinders may includean intake stroke injection performed at 240 deg BTDC, a compressionstroke injection performed at 40 deg BTDC, and a split ratio of 60/40(intake:compression) may be applied. In addition, spark timing may beretarded to 15 deg ATDC. In another example, during the VDE mode, thesplit fuel injection in the active cylinders may include an intakestroke injection performed at 230 deg BTDC, a compression strokeinjection performed at 35 deg BTDC, and a split ratio of 60/40(intake:compression) may be applied. In addition, spark timing may beretarded to 12 deg ATDC.

From 422, the routine returns to 418 to determine if non-VDE conditionshave been met, and accordingly reactivate the engine cylinders at 424.Optionally, split fuel injection may again be used during thereactivation, but this time in the reactivated cylinders, to improve thetransition out of the VDE mode, as discussed above at 426. As such, ifnon-VDE conditions are not met at 418, the routine includes maintainingVDE mode of engine operation at 428.

Returning to 420, in response to the temperature falling below the firstthreshold, as well as the second threshold (or a slower drop intemperature), the controller may infer that substantial catalyst coolinghas occurred and may reactivate all the engine cylinders at 424.Optionally, split fuel injection may again be used during thereactivation, but this time in the reactivated cylinders, to improve thetransition out of the VDE mode, as discussed above at 426. As elaboratedat FIG. 5, in response to monitored temperature falling below alight-off threshold, the controller may additionally compare the fuelpenalty associated with the split fuel injection and spark retard usage(at 422) with the fuel penalty associated with the reactivation of allthe engine cylinders (at 426). The controller may then, based on thecomparison, select the strategy that provides most fuel economy (orleast fuel penalty).

Now turning to FIG. 5, an example method 500 for temporarilytransitioning fuel injection of deactivated engine cylinders to a splitfuel injection is shown. The method allows engine operation in the VDEmode to be prolonged. The routine of FIG. 5 may be performed as part ofthe routine of FIG. 4, specifically at 422.

At 502, the routine includes confirming that the engine is in a VDEmode. Else the routine ends. As such, during the VDE mode, thetemperature of an emission control device may be monitored and isexpected to fall as the duration of VDE operation increases. Uponconfirming VDE mode, at 504, it may be determined by how much themonitored emission control device temperature has fallen below a firstthreshold. That is, a difference between the estimated temperature(Tcat) and the first threshold (Thr_1) may be determined.

At 506, an amount of spark retard required to raise the monitoredtemperature above the threshold temperature may be determined. Asdiscussed at FIG. 4, a controller may retard spark timing in the activecylinders in response to the monitored emission control devicetemperature falling below a (first) threshold. The amount of sparkretard applied may be based on a difference between the monitoredemission control device temperature and the (first) threshold (e.g., alight-off temperature), an amount of spark retard applied increased asthe difference increases.

It will be appreciated that in addition to the use of spark retard, theactive engine cylinders may also be operated with exhaust valve timingadjusted so as to maximize exhaust flow through the emission controldevice. For example, exhaust valve opening may be retarded. In oneexample, exhaust valve timing adjustments may be performed viacorresponding exhaust cam timing adjustments.

At 508, a fuel penalty associated with the calculated amount of sparkretard may be determined. At 510, a fuel penalty associated withreactivation of all engine cylinders to raise the exhaust temperaturemay be determined. At 512, the (first) fuel penalty associated with theuse of spark retard (FP_spk) may be compared to the (second) fuelpenalty associated with the cylinder reactivation (FP_reactvn).

In alternate embodiments, the routine may include estimating a fuelpenalty associated with the retarded spark timing, and determining ifthe fuel penalty is higher than a threshold penalty, wherein thethreshold penalty is based on a fuel penalty associated with cylinderreactivation.

If the fuel penalty associated with the use of spark retard (FP_spk) isless than the fuel penalty associated with the cylinder reactivation(FP_reactvn), then at 516, the routine includes operating the activecylinders with split fuel injection. Herein, the controller maydetermine that it is more fuel efficient to continue engine operation inthe VDE mode but with the active cylinders transiently shifted to use ofa split fuel injection and an amount of spark retard (than to transitionout of the VDE mode into the non-VDE mode by reactivating thedeactivated engine cylinders).

The controller may operate the active cylinders with fuel injected as atleast a first intake stroke injection and a second compression strokeinjection. As used herein, the first intake stroke injection includes afirst injection having one or more of a start time and an end timeduring an intake stroke, while the second compression stroke injectionincludes a second injection having one or more of a start time and anend time during a compression stroke. A split ratio of the split fuelinjection may be adjusted based on a difference between the monitoredtemperature and the threshold. Specifically, the adjusted split ratiomay include an increase in the first intake stroke injection amount anda corresponding decrease in the compression stroke injection amount asthe difference between the monitored temperature and the thresholdincreases. The split ratio may be further adjusted based on the amountof spark retard applied. For example, as the amount of spark retard usedincreases, the split ratio may be adjusted to decrease the amount offuel delivered in the intake stroke. The split ratio may also beadjusted based on an alcohol content of the injected fuel, with theadjusted split ratio including a decrease in the first intake strokeinjection amount and a corresponding increase in the compression strokeinjection amount as the fuel alcohol content increases.

In one example, during the VDE mode, the split fuel injection in theactive cylinders may include an intake stroke injection performed at 240deg BTDC, a compression stroke injection performed at 40 deg BTDC, and asplit ratio of 60/40 (intake:compression) may be applied. In addition,spark timing may be retarded to 15 deg ATSC.

As such, the use of split fuel injection and spark retard in the activecylinders may be continued for a number of combustion events followingthe drop in emission control device temperature. Then, when thetemperature has been returned to or above the first threshold, the splitfuel injection and spark retard usage may be discontinued. Thereafter,fuel injection of the active engine cylinders may be adjusted based onengine operating conditions including engine speed and load. As such,this may include resuming fueling in a single injection mode (forexample, as a single intake stroke injection) at 518. Alternatively, asplit fuel injection may be applied, as required.

If the fuel penalty associated with the use of spark retard (FP_spk) ismore than the fuel penalty associated with the cylinder reactivation(FP_reactvn), or if the fuel penalty associated with the use of sparkretard is higher than the threshold penalty associated with cylinderreactivation, at 514, the routine includes reactivating the one or moredeactivated cylinders responsive to the temperature falling below thethreshold, and transitioning out of the VDE mode of engine operation.Herein, VDE of engine operation is discontinued and split injection ofthe active cylinders is not performed. An example adjustment of activeVDE cylinders is shown at FIG. 7.

In this way, a split fuel injection strategy combined with usage ofspark retard can be used in active cylinders during selected VDE modeoperating conditions to maintain an exhaust catalyst above an activationtemperature. In doing so, fuel economy benefits from the continued useof cylinder deactivation can be achieved.

As an example, a method for an engine may comprise selectivelydeactivating one or more engine cylinders responsive to operatingconditions and monitoring a temperature of an exhaust catalyst coupleddownstream of the deactivated cylinders. During a first drop in exhaustcatalyst temperature, the method includes operating active cylinderswith split fuel injection and retarded spark timing while during asecond, different drop in exhaust catalyst temperature, the methodincludes reactivating the one or more deactivated engine cylinders.Herein, the first drop may include a first drop in exhaust catalysttemperature to below a first threshold while the second drop includes asecond drop in exhaust catalyst temperature to a second threshold lowerthan the first threshold. In an alternate example, the first drop mayoccur at a higher drop rate than the second drop. Further still, thefirst drop may include a smaller spark retard fuel penalty while thesecond drop includes a larger spark retard fuel penalty. During thesecond drop, the controller may operate the engine with exhaust valveopening retarded via adjustments to an exhaust cam timing.

In another example, an engine system comprises an engine with each of afirst and second group of cylinders; a fuel injector coupled to eachengine cylinder; an emission control device coupled downstream of eachof the first and second group of cylinders; and a temperature sensorconfigured to estimate a temperature of the emission control device. Thesystem may further include a controller including instructions for,selectively deactivating the second group of cylinders while maintainingthe first group of cylinders active responsive to operating conditions;and during the deactivation, in response to a drop in temperature of theemission control device, maintaining the second group of cylindersdeactivated while shifting fuel injection of the first group ofcylinders from single fuel injection to split fuel injection.

Herein, shifting fuel injection from single fuel injection to split fuelinjection includes shifting fuel injection from a single intake strokeinjection to a first intake stroke injection and a second compressionstroke injection, a ratio of the first injection amount to the secondinjection amount based on the temperature of the emission controldevice. The controller may include further instructions for, whileshifting the fuel injection, operating the first group of cylinders withspark timing retarded, an amount of spark retard applied based on thedrop in temperature of the emission control device. The controller mayinclude still further instructions for, while shifting the fuelinjection, operating the first group of cylinders with exhaust valveopening retarded, an amount of exhaust valve opening retard appliedbased on the drop in temperature of the emission control device.

Now turning to FIG. 6, an example method 600 for temporarilytransitioning fuel injection of reactivated engine cylinders to a splitfuel injection is shown. The method allows restart combustion stabilitywhen transitioning out of a VDE mode to be improved. The routine of FIG.6 may be performed as part of the routine of FIG. 4, specifically at426.

At 602, the routine includes confirming that the engine is in a VDE modeand that non-VDE conditions have been met. Else the routine ends. Uponconfirming that cylinder reactivation conditions have been met, at 604,a duration of the previous deactivation (that is, a duration ofoperation in the VDE mode) may be determined. In addition, a change inexhaust catalyst temperature over the duration may also be determined.In some embodiments, the controller may also determine an amount of EGRdilution being used in the active cylinders prior to the reactivation.

At 606, an amount of spark retard required to raise the exhaust catalysttemperature upon cylinder reactivation may be determined. That is, anamount of spark retard that has to be applied to the reactivatedcylinder upon reentry into non-VDE mode may be determined. In someengine configurations, such as those where an entire bank is deactivatedduring the VDE mode, a temperature of the exhaust catalyst coupled tothe inactive bank may fall over the duration of the cylinderdeactivation due to the lack of exhaust heat generated from thedeactivated cylinders. As discussed at FIG. 4, a controller may retardspark timing in the reactivated cylinders to raise the exhaust catalysttemperature to or above a light-off temperature to ensure catalytictreatment of exhaust emissions.

At 608, a fuel injection mode to be used on the reactivated cylindersmay be determined based at least on the duration of deactivation, thedrop in exhaust catalyst temperature, and the EGR dilution appliedduring the deactivation. Specifically, it may be determined if a singlefuel injection or a split fuel injection is required. In one example, ifthe duration of deactivation is larger and/or a higher drop in exhaustcatalyst temperature (of the inactive bank) has occurred over theduration, then use of a split fuel injection with spark retarded by anamount may be used to expedite exhaust catalyst warming. In comparison,if the duration of deactivation is smaller and/or a lower drop inexhaust catalyst temperature has occurred over the duration, then use ofa single fuel injection with no spark retard may be sufficient to warmthe exhaust catalyst.

The selection may be further based on the determination of combustionstability. For example, if low combustion stability is likely during thereactivation, the split injection may be used. This may be the case whenhigher EGR dilution was used during the preceding deactivation and lowerEGR dilution is required upon reactivation. As previously discussed, thehigher cylinder load during the VDE conditions allows for the use of ahigher EGR rate. Upon reactivation, the cylinder load may drop, reducingthe cylinder's EGR tolerance and therefore requiring EGR rates to bereduced (at least until engine speed has reached a threshold speed afterwhich EGR operation can be resumed). In one example, during thereactivation, no EGR may be desired. Thus, during the reactivation, EGRmay be ramped out by closing (or reducing an opening) of the EGR valve.However, due to the long transport delay associated with the EGRcircuit, the EGR may ramp down at a slower rate than desired, resultingin cylinders operating at low load and high EGR dilution conditionswhere they are prone to combustion instability and misfires. Duringthese conditions, the transient shift to a split fuel injection mayimprove the combustion stability and EGR tolerance of the cylinders.Thus, during conditions where the cylinders were operated with a higheramount of EGR (e.g., higher than a threshold) during the VDE mode, splitinjection may be selected during the subsequent reactivation or shift tonon-VDE mode. In contrast, during conditions where the cylinders wereoperated with a lower amount of EGR (e.g., lower than a threshold)during the VDE mode, single injection may be selected during thesubsequent reactivation or shift to non-VDE mode.

At 610, it may be confirmed that a split injection mode has beenselected. If a split injection mode has not been selected, at 612, theroutine reactivates all engine cylinders and operates the engine withfuel delivered as a single intake stroke injection. In addition, sparktiming may be retarded, as required, to heat the exhaust catalyst of theinactive bank.

Upon confirmation that a split injection mode has been selected, at 614,the routine includes reactivating the previously deactivated enginecylinders and operating the reactivated cylinders with split fuelinjection while maintaining single fuel injection in the previouslyactive cylinders. As used herein, operating the reactivated cylinderswith split fuel injection includes operating the cylinders with at leasta first intake stroke injection and a second compression strokeinjection while maintaining a single intake stroke injection of fuel inthe cylinders that were active during the preceding VDE mode.

The first intake stroke injection may include a first injection havingone or more of a start time and an end time during an intake stroke,while the second compression stroke injection may include a secondinjection having one or more of a start time and an end time during acompression stroke. A split ratio of the split fuel injection may bebased on the estimated duration of the selective deactivation, the splitratio adjusted to decrease an amount of the first intake strokeinjection while correspondingly increasing an amount of the secondcompression stroke injection as the duration of selective deactivationincreases. The split ratio may be further adjusted based on theestimated exhaust catalyst temperature, the split ratio adjusted toincrease an amount of the first intake stroke injection whilecorrespondingly decreasing an amount of the second compression strokeinjection as the catalyst temperature falls below a threshold. Theinjection timings, split ratio, and spark timing applied to thepreviously deactivated bank of cylinders allows the catalyst to quicklyobtain an efficient operating temperature while still maintainingreasonable combustion stability.

The split ratio of the reactivated cylinders may be further adjustedbased on the estimated EGR dilution before the reactivation. Inparticular, when the EGR dilution is higher, a timing of the fuelinjection may be adjusted to provide a larger portion of the fuel duringthe intake stroke (e.g., as a homogenous lean intake stroke injection)and a smaller portion of the fuel during the compression stroke (e.g.,as a rich stratified compression stroke injection). As discussed above,when transitioning from the VDE mode to the non-VDE mode, individualcylinder loads may decrease based on the decrease in aircharge. Thelighter cylinder loads generally have less stable combustion and theinteraction with the transient fuel compensation, and other cylinderconditions that are different than the operating cylinders due tocooling during deactivation may contribute to less stable combustionduring reactivation. The EGR may continue to interfere with the lightercylinder load until the EGR delivered to the cylinders has beensufficiently bled down to reduce combustion issues. While charge motioncontrol valves (CMCVs) can be used to adjust the in-cylinder motion ofthe air fuel mixture delivered to the cylinder during the transition(due to the better mixing and more stable combustion), the slowerresponse time of the CMCV (e.g., the CMCV not shutting quickly enoughwhen transitioning to the lower cylinder load), combustion stability canbecome compromised leading to slow burns or even misfires during thereactivation. Thus, by using a split fuel injection, the combustionstability at low cylinder load can be improved. In one example, theportion of fuel delivered during the compression stroke may correspondto the minimum flow mass of the injector. By adjusting the timing of thecompression injection to coincide with spark timing (or performing thecompression injection immediately before or after the spark event), astratified charge combustion can be used to reduce cylinder burn times.In addition, the stratified combustion may enhance oxidation in thecatalyst, and further improve the catalyst reheating.

As such, one or more of the split ratio, timing and a pressure of thefuel injection may be further adjusted based on an alcohol content ofthe injected fuel. For example, the amount of second compression strokeinjection may be increased and the amount of the first intake strokeinjection may be correspondingly decreased as the alcohol content of theinjected fuel increases.

In addition to the split fuel injection, during the reactivation, sparktiming may be retarded in the reactivated cylinders to expedite exhaustcatalyst heating. Specifically, an amount of spark retard may beadjusted based on the exhaust catalyst temperature (e.g., based on adifference between the exhaust catalyst temperature and a light-offtemperature or alternate threshold temperature).

In one example, the split fuel injection in the reactivated cylindersmay include an intake stroke injection performed at 240 deg BTDC, acompression stroke injection performed at 40? deg BTDC, and a splitratio of 60/40 (intake:compression) may be applied. In addition, sparktiming may be retarded to 15 deg ATDC.

As such, the operating of the reactivated cylinders with split fuelinjection may be performed for a number of combustion events since thereactivation. For example, until an engine speed is at or above athreshold speed (e.g., idling speed) and/or until the EGR rate is belowa threshold (e.g., EGR fully purged). Then, at 616, fueling in thesingle injection mode may be resumed. For example, after the number ofcombustion events has elapsed, the controller may operate thereactivated cylinders with fuel injected as a single fuel injection inthe intake stroke.

The number of combustion events may be based on an engine load duringthe cylinder reactivation, the number of combustion events increaseduntil the engine load reaches steady-state conditions. The number ofcombustion events may be further based on a duration of selectivedeactivation, an exhaust catalyst temperature, and an EGR level duringthe reactivation, the number of combustion events increased as theduration increases, the exhaust catalyst temperature decreases and theEGR level increases.

It will be appreciated that during the reactivation, while thereactivated cylinders are operated with split fuel injection, thealready active cylinders may be operated with fuel injection adjustedbased on engine speed-load conditions. For example, the active cylindersmay be operated with single fuel injection for the number of combustionevents, the single fuel injection including fuel injected as a singleintake stroke injection.

In this way, by operating reactivated cylinders with split fuelinjection for a number of combustion events during a reactivation fromVDE mode of engine operation, restart combustion stability of thecylinders is improved.

As an example, a method for an engine comprises, selectivelydeactivating one or more engine cylinders in response to operatingconditions; and during reactivation, operating reactivated cylinderswith fuel injected as each of an intake stroke injection and acompression stroke injection for a number of combustion events since thereactivation. A ratio of fuel injected in the intake stroke injectionrelative to the compression stroke injection may be based on atemperature of an exhaust catalyst, an amount of fuel injected in theintake stroke increased as the temperature of the exhaust catalyst fallsbelow a threshold. Herein, selectively deactivating one or more enginecylinders includes deactivating one or more engine cylinders of a firstengine bank, the engine including a second bank, and wherein the ratioof fuel injected is based on a temperature of an exhaust catalystcoupled to the first bank and not the second bank. The split ratio offuel injected may be further based on an alcohol content of the injectedfuel, an amount of fuel injected in the intake stroke injectiondecreased and an amount of fuel injected in the compression strokeinjection correspondingly increased as the alcohol content of theinjected fuel increases.

In another example, a method for an engine comprises selectivelydeactivating one or more engine cylinders. Then, during a firstreactivation, the method includes operating the reactivated cylinderswith fuel injected as a single injection. In comparison, during a secondreactivation, the routine includes operating the reactivated cylinderswith fuel injected as a split injection with fuel injected as each of anintake stroke injection and a compression stroke injection. Herein thesecond reactivation occurs after a longer duration of deactivation whilethe first reactivation occurs after a shorter duration of deactivation.The method further includes, during the first reactivation, maintainingspark timing of the reactivated cylinders, and during the secondreactivation, retarding spark timing of the reactivated cylinders.

Now turning to FIG. 7, map 700 depicts an example fuel injectionadjustment for active engine cylinders. The adjustment is performedresponsive to a drop in exhaust catalyst temperature to allow cylinderdeactivation to be prolonged. Map 700 depicts an engine mode ofoperation (VDE or non-VDE) at plot 702, fuel injection profile of afirst engine bank at plot 704, fuel injection profile of a second enginebank at plot 706, spark timing of the first engine bank at plot 708,spark timing of the second engine bank at plot 709, and an exhaustcatalyst temperature of an catalyst coupled to the second bank (the bankthat is inactivated during the VDE mode) at plot 710.

Prior to t1, the engine may be operating in a non-VDE mode (plot 702)with all cylinders on each bank firing. During the non-VDE mode ofoperation, cylinders in both the first and second engine bank may bereceiving fuel (plots 704-706) as a single intake stroke injection(depicted by single solid bar). At t1, in response to a change inoperating conditions (e.g., a drop in engine load or torque demand), theengine may shift to a VDE mode of operation. Specifically, all cylinderson the second engine bank may be selectively deactivated by shutting offuel and spark (plot 709) while all cylinders on the first engine bankremain active (plot 704 and 708). As such, due to the cylinderdeactivation, the cylinder load of the active cylinders on the firstengine bank may increase. As shown, the active cylinders on the firstengine bank may continue single intake stroke injection (with a largeramount of total fuel injected corresponding to the higher cylinderload).

Between t1 and t2, as engine operation with cylinder deactivationcontinues, there may be a drop in temperature at the exhaust catalysttemperature coupled downstream of the inactive bank such that at t2, theexhaust catalyst is at or below a threshold temperature 712. For thedeactivated bank, with deactivated valves, there will be no airflowthrough the bank, but the catalyst will cool since no combusted air andfuel mixture is heating the catalyst. As such, this drop in exhaustcatalyst temperature may not only lead to a preponing of cylinderreactivation, but also lead to a fuel penalty during the cylinderreactivation because of the extra heat that needs to be generated toheat the catalyst. Thus, to reduce the fuel penalty and allow the engineto remain in the VDE more for a longer time, at t2, while maintainingthe second bank deactivated, fuel injection of the active cylinders onthe first bank may be shifted to a split fuel injection (plot 704).Specifically, the total amount of fuel may be delivered as a firstintake stroke injection (depicted by solid bar) and a second compressionstroke injection (depicted by hatched bar). Split fuel injection timing,ratio, and pressure may be adjusted based at least on a differencebetween the exhaust catalyst temperature and the threshold. In thedepicted example, the split ratio is adjusted to include a higher amountof the first intake stroke injection and a lower amount of the secondcompression stroke injection. In addition to the split fuel injection,spark timing may be retarded from MBT in the active cylinders (plot708), the spark retard adjusted to raise the exhaust temperaturesufficiently so that the exhaust catalyst can be warmed.

The use of split fuel injection alongside spark retard in the activecylinders is continued for a number of combustion events between t2 andt3 until the catalyst temperature is returned at or above threshold 712.At t3, once the catalyst has been sufficiently warmed, single intakestroke fuel injection in the active cylinders is resumed while spark isreturned to MBT. At t4, in response to reactivation conditions being met(e.g., a rise in engine load or torque demand), the deactivatedcylinders of the second bank may be reactivated with fuel and sparkreturned to the cylinders.

As such, if the active cylinders were not shifted transiently to a splitfuel injection mode, the second engine bank may have needed to bereactivated much earlier (as shown at dotted line 703), specifically att2, in response to the drop in exhaust catalyst temperature. Therein,fuel injection (see 705) and spark (see 707) may be returned to thesecond bank at t2. As such, this may reduce the fuel economy benefits ofVDE operation by cutting short the duration of engine operation in VDEmode. Thus, by shifting the active cylinders to a split fuel injection,VDE operation can be prolonged and fuel economy benefits can be extendedfor a longer duration of engine operation.

While FIG. 7 shows spark timing on the active bank (bank_1) at MBTbetween t1 and t2, it will be appreciated that in alternate examples,between t1 and t2, spark timing on the active bank may be retarded fromMBT due to operating at higher cylinders loads and protection fromborderline limits. However, when used, the level of spark retard usedduring the VDE mode when catalyst heating is not required (as between t1and t2) would not be as large as the spark retard amount used duringcatalyst heating (as depicted between t2 and t3).

It will also be appreciated that the engine configuration used in theexample of FIG. 7 may correspond to an engine where all engine cylindersof a bank are deactivated, such as in engines with flat crankshaftarrangement and even firing order (e.g., V6 or V10 engines). Inalternate engine configurations, such as where the engine has unevenfiring order (e.g., a V8 where at a given time, the outer or innercylinders of a bank are deactivated), the controller may monitor thetemperature of an exhaust catalyst coupled to each bank and in responseto cooling on any given bank, the active cylinders of that bank may beshifted to a split fuel injection. For example, during a VDE mode, afirst group of cylinders of a first bank and a first group of cylindersof a second bank may be deactivated while a second group of cylinders ofthe first bank and a second group of cylinders of the second bank remainactive. In response to a drop in exhaust catalyst temperature at thefirst bank (but not second bank), during the VDE mode, the second groupof cylinders of the first bank may be shifted to a split fuel injection,while the active cylinders of the second bank are maintained operatingwith single fuel injection. Likewise, in response to a drop in exhaustcatalyst temperature at the second bank (but not first bank), during theVDE mode, the second group of cylinders of the second bank may beshifted to a split fuel injection, while the active cylinders of thefirst bank are maintained operating with single fuel injection. Thisallows engine operation in the VDE mode to be prolonged.

Now turning to FIG. 8, map 800 depicts an example fuel injectionadjustment for reactivated engine cylinders. The adjustment is performedduring a transition out of VDE mode to improve restart combustionstability and transient torque disturbances. Map 800 depicts an enginemode of operation (VDE or non-VDE) at plot 802, fuel injection profileof a first engine bank at plot 804, fuel injection profile of a secondengine bank at plot 806, spark timing of the first engine bank at plot808, spark timing of the second engine bank at plot 809, and atemperature of an exhaust catalyst temperature coupled to the inactivebank at plot 810.

Prior to t1, the engine may be operating in a non-VDE mode (plot 802)with all cylinders on each bank firing. During the non-VDE mode ofoperation, cylinders in both the first and second engine bank may bereceiving fuel (plots 804-806) as a single intake stroke injection(depicted by single solid bar). In addition, spark may be at a nominaltiming, such as at MBT. At t1, in response to a change in operatingconditions (e.g., a drop in engine load or torque demand), the enginemay shift to a VDE mode of operation. Specifically, cylinders on thesecond engine bank may be selectively deactivated by shutting of fueland spark (plots 806 and 809) while cylinders on the first engine bankremain active (plots 804 and 808). As such, due to the cylinderdeactivation, the cylinder load of the active cylinders may increase. Asshown, the active cylinders on the first engine bank may continue singleintake stroke injection (with a larger amount of total fuel injectedcorresponding to the higher cylinder load).

Between t1 and t2, as engine operation with cylinder deactivationcontinues, there is a gradual drop in exhaust catalyst temperature dueto the lack of a hot combusted air-fuel mixture flowing over the exhaustcatalyst, since the entire bank coupled to the catalyst is inactive. Att2, the exhaust catalyst may be close to threshold temperature 812 andengine reactivation conditions may be met. For example, the engine loadand torque demand may increase. When transitioning from the VDE mode tothe non-VDE mode, individual cylinder loads may decreases based on thedecrease in aircharge. The lighter cylinder loads generally have lessstable combustion and the interaction with the transient fuelcompensation, and other cylinder conditions that are different than theoperating cylinders due to cooling during deactivation may contribute toless stable combustion during reactivation.

To overcome the issues related to the exhaust catalyst activation andalso to improve restart combustion stability, at t2, during thereactivation, while maintaining the single injection of the first bank,fuel injection of the reactivated cylinders on the second bank may beshifted to a split fuel injection (plot 806). Specifically, the totalamount of fuel may be delivered as a first intake stroke injection(depicted by solid bar) and a second compression stroke injection(depicted by hatched bar). Split fuel injection timing, ratio, andpressure may be adjusted based at least on a difference between theexhaust catalyst temperature and the threshold. In the depicted example,the split ratio is adjusted to include a higher amount of the firstintake stroke injection and a lower amount of the second compressionstroke injection. In addition to the split fuel injection, spark timingmay be retarded from MBT in the reactivated cylinders (plot 809), thespark retard adjusted to raise the exhaust temperature sufficiently sothat the exhaust catalyst can be warmed. The spark timing of the firstengine bank may be maintained at MBT.

The use of split fuel injection alongside spark retard in thereactivated cylinders is continued for a number of combustion eventsbetween t2 and t3 until the catalyst temperature is raised, and theengine speed has increased to a level where combustion stability is notdegraded. For example, the split injection may be continued at leastuntil the engine speed is at or above an idle speed. At t3, once thecatalyst has been sufficiently warmed, and the engine speed has reachedan idle speed, single intake stroke fuel injection in the reactivatedcylinders is resumed while spark is returned to MBT.

While FIG. 8 shows spark timing on the active bank (bank_1) at MBTbetween t1 and t2, it will be appreciated that in alternate examples,between t1 and t2, spark timing on the active bank may be retarded fromMBT due to operating at higher cylinders loads and protection fromborderline limits. However, when used, the level of spark retard usedduring the VDE mode when catalyst heating is not required (as between t1and t2) would not be as large as the spark retard amount used on thereactivated bank (bank_2) during catalyst heating (as depicted betweent2 and t3).

It will also be appreciated that the engine configuration used in theexample of FIG. 8 may correspond to an engine where all engine cylindersof a bank are deactivated, such as in engines with flat crankshaftarrangement and even firing order (e.g., V6 or V10 engines). Inalternate engine configurations, such as where the engine has unevenfiring order (e.g., a V8 where at a given time, the outer or innercylinders of a bank are deactivated), the controller may monitor thetemperature of an exhaust catalyst coupled to each bank and in responseto cooling on any given bank, the reactivated cylinders of that bank maybe shifted to a split fuel injection. For example, during a VDE mode, afirst group of cylinders of a first bank and a first group of cylindersof a second bank may be deactivated while a second group of cylinders ofthe first bank and a second group of cylinders of the second bank remainactive. In response to a drop in exhaust catalyst temperature at thefirst bank (but not second bank), the engine may be shifted to a non-VDEmode and during the reactivation, at least the first group of cylindersof the first bank may be shifted to a split fuel injection, while thecylinders of the second bank are maintained operating with single fuelinjection. Optionally, based on the exhaust catalyst temperature drop,during the reactivation, all cylinders of the first bank may be shiftedto a split fuel injection. Likewise, in response to a drop in exhaustcatalyst temperature at the second bank (but not first bank), the enginemay be shifted to a non-VDE mode and during the reactivation, at leastthe first group of cylinders of the second bank may be shifted to asplit fuel injection, while the cylinders of the first bank aremaintained operating with single fuel injection. Optionally, based onthe exhaust catalyst temperature drop, during the reactivation, allcylinders of the second bank may be shifted to a split fuel injection.This improves cylinder restartability.

Now turning to FIG. 9, map 900 depicts an example fuel injectionadjustment for reactivated engine cylinders. The adjustment is performedduring a transition out of VDE mode to improve restart combustionstability and transient torque disturbances. Map 900 depicts an enginemode of operation (VDE or non-VDE) at plot 902, fuel injection profileof a first engine bank at plot 904, fuel injection profile of a secondengine bank at plot 906, spark timing of the first engine bank at plot908, spark timing of the second engine bank at plot 909, and an EGRlevel (of all active cylinders, during VDE or non-VDE mode) at plot 910.

Prior to t1, the engine may be operating in a non-VDE mode (plot 902)with all cylinders on each bank firing due to engine load being higherthan a threshold. During the non-VDE mode of operation, cylinders inboth the first and second engine bank may be receiving fuel (plots904-906) as a single intake stroke injection (depicted by single solidbar). In addition, spark (plot 908-909) may be at a nominal timing, suchas at MBT. Further, due to the higher engine load conditions, EGR may beprovided to the engine cylinders as shown (plot 910, before t1, showsthe EGR level in all engine cylinders) by increasing the opening of anEGR valve. In one example, the EGR provided may be low-pressure EGR.Further, the EGR may be provided at a fixed rate (that is, at a fixedpercentage relative to airflow). In another example, the LP-EGR may beprovided at a variable rate relative to airflow. In still furtherexamples, EGR may be provided as a combination of low-pressure andhigh-pressure EGR.

At t1, in response to a change in operating conditions (e.g., a drop inengine load or torque demand), the engine may shift to a VDE mode ofoperation. Specifically, cylinders on the second engine bank may beselectively deactivated by shutting of fuel and spark (plots 906 and909) while cylinders on the first engine bank remain active (plots 904and 908). As shown, the active cylinders on the first engine bank maycontinue single intake stroke injection (with a larger amount of totalfuel injected corresponding to the higher cylinder load).

As such, due to the cylinder deactivation, the cylinder load of theactive cylinders may increase. This improves their EGR tolerance andallows for a higher level of EGR to be used in the active cylindersduring the VDE mode of operation (plot 910, between t1 and t2, shows theEGR level of the active cylinders). By using EGR in addition to VDE,further fuel economy benefits are achieved. Furthermore, EGR benefitscan be extended to the low engine load conditions due to the elevatedcylinder load during the VDE mode of operation.

At t2, in response to a change in operating conditions (e.g., anincrease in engine load or torque demand), the engine may shift back toa non-VDE mode of operation. Specifically, cylinders on the secondengine bank may be selectively reactivated by returning fuel and spark.Due to the cylinder reactivation, the cylinder load of the activecylinders may decrease. The lighter cylinder loads generally have lessstable combustion and the interaction with the transient fuelcompensation, and other cylinder conditions during deactivation maycontribute to less stable combustion during reactivation. In addition,the lower cylinder load reduces their EGR tolerance. Thus, at t2, an EGRvalve may be closed and EGR may be ramped out during the reactivation.However, due to long transport delays along the EGR passage, the actualEGR ramp out rate (plot 910) may be slower than the desired ramp-outrate (dashed segment 911). In particular, the EGR may continue tointerfere with the lighter cylinder load until the EGR delivered to thecylinders has been sufficiently bled down to reduce combustion issues.While charge motion control valves (CMCVs) can be used to adjust thein-cylinder motion of the air fuel mixture delivered to the cylinderduring the transition (due to the better mixing and more stablecombustion), due to the slower response time of the CMCV (e.g., the CMCVnot shutting quickly enough when transitioning to the lower cylinderload), combustion stability can become compromised leading to slow burnsor even misfires during the reactivation.

To overcome these issues and improve restart combustion stability, att2, during the reactivation, fuel injection of all the cylinders,including the reactivated cylinders on the second bank as well as theactive cylinders of the first bank, may be shifted to a split fuelinjection (plot 906). Specifically, the total amount of fuel may bedelivered as a first intake stroke injection (depicted by solid bar) anda second compression stroke injection (depicted by hatched bar). Splitfuel injection timing, ratio, and pressure may be adjusted based atleast on the EGR dilution present in the engine system prior to thereactivation (that is, during the immediately preceding deactivation).In the depicted example, the split ratio is adjusted to include a higheramount of the first intake stroke injection and a lower amount of thesecond compression stroke injection. In addition to the split fuelinjection, spark timing may be retarded from MBT in the reactivatedcylinders (plot 909), the spark retard adjusted to raise the exhausttemperature sufficiently so that the exhaust catalyst on the previouslyinactive bank be warmed. At the same time, the spark timing of the firstengine bank may be maintained at MBT.

It will be appreciated that in example of FIG. 9, split injection isused in all engine cylinders during reactivation until the EGR in theintake system has bled down because EGR is delivered to all the enginecylinders via a common EGR passage. However, there may be alternateengine configurations wherein the EGR system is configured to deliverEGR to distinct sets of cylinders via distinct passages. In those engineconfigurations, it may be possible to deliver EGR to only the activecylinders during the VDE mode of operation, in which case during thesubsequent cylinder reactivation, split fuel injection may be used inonly the previously active cylinders until the EGR has sufficientlypurged, while the reactivated cylinders are maintained in single fuelinjection.

The use of split fuel injection is continued on both engine banks (inthe depicted example) for a number of combustion events between t2 andt3 until the EGR is sufficiently purged. At t3, once the EGR has droppedto a sufficiently low level, single intake stroke fuel injection in thereactivated cylinders is resumed. Also, spark timing in the reactivatedengine bank may be returned to MBT.

It will be appreciated that while the example of FIG. 9 shows sparktiming retarded between t2 and t3, in alternate examples, spark retardon the reactivated cylinders may be adjusted based on the exhaustcatalyst temperature. Also, while FIG. 9 shows spark timing on theactive bank (bank_1) at MBT between t1 and t2, it will be appreciatedthat in alternate examples, between t1 and t2, spark timing on theactive bank may be retarded from MBT due to operating at highercylinders loads and protection from borderline limits. However, whenused, the level of spark retard used during the VDE mode (as between t1and t2) would not be as large as the spark retard amount used on thereactivated bank (bank_2) for catalyst heating (as depicted between t2and t3).

In one example, a method for an engine comprises selectivelydeactivating one or more engine cylinders and operating active cylinderswith EGR. Then, during reactivation, the method includes operating allengine cylinders with split fuel injection until EGR is less than athreshold. The split fuel injection includes a first lean homogeneousintake stroke injection and a second rich stratified compression strokeinjection. A split ratio of the split fuel injection is based on one ormore of the EGR during the selective deactivation, and an exhaustcatalyst temperature during the reactivation. Operating active cylinderswith EGR includes operating active cylinders with a fixed percentage ofEGR relative to air flow. Operating active cylinders with EGR alsoincludes adjusting an EGR level of the active cylinders based on averagecylinder load of the active cylinders during the selective deactivation,the EGR level increased as the average cylinder load increases. Further,during the reactivation, reactivated cylinders are operated with sparktiming retard, the retard based on exhaust catalyst temperature.

Thus by operating all engine cylinders with split fuel injection uponcylinder reactivation until an EGR level of the intake system is below athreshold, combustion stability issues associated with high EGR dilutionat low cylinder loads can be mitigated. In addition, the use of splitinjection expedites exhaust catalyst reactivation following the VDE modeof operation, improving exhaust emissions. By further adjusting thesplit ratio based on an alcohol content of the injected fuel,drivability deterioration from mixed fuel usage is reduced. Inparticular, poor combustion events that may result in a stumble can bereduced.

In this way, by operating reactivated cylinders with split fuelinjection with a portion of fuel delivered during an intake stroke and aportion of the fuel delivered during a compression stroke, restartcombustion stability of the cylinders is improved. In particular,combustion stability issues associated with the reduction in cylinderload upon cylinder reactivation can be mitigated. In addition, the splitinjection expedites exhaust catalyst reactivation following the VDE modeof operation, improving exhaust emissions. By further adjusting thesplit ratio based on an alcohol content of the injected fuel,drivability deterioration from mixed fuel usage is reduced. Inparticular, poor combustion events that may result in a stumble can bereduced.

By also operating active cylinders with split fuel injection for anumber of combustion events during a VDE mode of engine operation, thetemperature and catalytic efficiency of an exhaust catalyst can berapidly recovered while also improving combustion stability during thetransition to a non-VDE mode of operation. By using the split injectionto expediate exhaust warming, a duration of engine operation in the VDEmode is prolonged. This allows the fuel economy benefits of cylinderdeactivation to be extended. Overall, engine performance is improved.

Note that the example control and estimation routines included hereincan be used with various engine and/or vehicle system configurations.The specific routines described herein may represent one or more of anynumber of processing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, various actions,operations, and/or functions illustrated may be performed in thesequence illustrated, in parallel, or in some cases omitted. Likewise,the order of processing is not necessarily required to achieve thefeatures and advantages of the example embodiments described herein, butis provided for ease of illustration and description. One or more of theillustrated actions, operations and/or functions may be repeatedlyperformed depending on the particular strategy being used. Further, thedescribed actions, operations and/or functions may graphically representcode to be programmed into non-transitory memory of the computerreadable storage medium in the engine control system.

It will be appreciated that the configurations and routines disclosedherein are exemplary in nature, and that these specific embodiments arenot to be considered in a limiting sense, because numerous variationsare possible. For example, the above technology can be applied to V-6,I-4, I-6, V-12, opposed 4, and other engine types. The subject matter ofthe present disclosure includes all novel and non-obvious combinationsand sub-combinations of the various systems and configurations, andother features, functions, and/or properties disclosed herein.

The following claims particularly point out certain combinations andsub-combinations regarded as novel and non-obvious. These claims mayrefer to “an” element or “a first” element or the equivalent thereof.Such claims should be understood to include incorporation of one or moresuch elements, neither requiring nor excluding two or more suchelements. Other combinations and sub-combinations of the disclosedfeatures, functions, elements, and/or properties may be claimed throughamendment of the present claims or through presentation of new claims inthis or a related application. Such claims, whether broader, narrower,equal, or different in scope to the original claims, also are regardedas included within the subject matter of the present disclosure.

The invention claimed is:
 1. A method for an engine, comprising:selectively deactivating one or more engine cylinders responsive tooperating conditions; and during reactivation of the cylinders,operating the reactivated cylinders with split fuel injection; andretarding spark timing in the reactivated cylinders, an amount of sparkretard based on an exhaust catalyst temperature.
 2. The method of claim1, wherein operating the reactivated cylinders with split fuel injectionincludes operating the reactivated cylinders with split fuel injectionfor a number of combustion events and then resuming single fuelinjection.
 3. The method of claim 2, wherein operating the reactivatedcylinders with split fuel injection includes operating with at least afirst intake stroke injection and a second compression stroke injection.4. The method of claim 3, wherein the first intake stroke injection is alean homogeneous intake stroke injection and wherein the secondcompression stroke injection is a rich stratified compression strokeinjection.
 5. The method of claim 4, wherein a split ratio of the splitfuel injection is based on a duration of the selective deactivation, thesplit ratio adjusted to decrease an amount of the first intake strokeinjection while correspondingly increasing an amount of the secondcompression stroke injection as the duration of selective deactivationincreases.
 6. The method of claim 5, wherein the split ratio is furtherbased on the exhaust catalyst temperature, the split ratio adjusted toincrease the amount of the first intake stroke injection whilecorrespondingly decreasing the amount of the second compression strokeinjection as the catalyst temperature falls below a threshold.
 7. Themethod of claim 6, wherein one or more of the split ratio, a timing, anda pressure of the fuel injection is further adjusted based on an alcoholcontent of injected fuel, the amount of the second compression strokeinjection increased and the amount of the first intake stroke injectioncorrespondingly decreased as the alcohol content of the injected fuelincreases.
 8. The method of claim 1, wherein the split fuel injection isadjusted based on an EGR level of active cylinders during the selectivedeactivation, the split fuel injection continued until the EGR levelduring the reactivation is below a threshold EGR rate.
 9. The method ofclaim 1, wherein the number of combustion events is based on an engineload during the cylinder reactivation, the number of combustion eventsincreased until the engine load reaches steady-state conditions.
 10. Themethod of claim 9, wherein the number of combustion events is furtherbased on a duration of selective deactivation, an engine coolanttemperature, and an EGR level during the reactivation, the number ofcombustion events increased as the duration increases, the enginecoolant temperature decreases and the EGR level increases.
 11. Themethod of claim 1, further comprising, during the reactivation,operating the already active cylinders with fuel injected for a numberof combustion events as a single intake stroke fuel injection.
 12. Themethod of claim 2, further comprising, after the number of combustionevents, operating the reactivated cylinders with fuel injected as asingle fuel injection in an intake stroke.
 13. A method for an engine,comprising: selectively deactivating one or more engine cylinders inresponse to operating conditions; and during reactivation, operatingreactivated cylinders with fuel injection split into each of an intakestroke injection and a compression stroke injection for a number ofcombustion events since the reactivation, a ratio of fuel injected inthe intake stroke injection relative to the compression stroke injectionbased on an exhaust catalyst temperature, an amount of fuel injected inthe intake stroke injection increased as the exhaust catalysttemperature falls below a threshold.
 14. The method of claim 13, whereinselectively deactivating one or more engine cylinders includesdeactivating one or more engine cylinders of a first engine bank, theengine including a second bank, and wherein the ratio of fuel injectedis based on a temperature of an exhaust catalyst coupled to the firstbank and not the second bank.
 15. The method of claim 13, wherein theratio of fuel injected is further based on an alcohol content of theinjected fuel, the amount of fuel injected in the intake strokeinjection decreased, and an amount of fuel injected in the compressionstroke injection correspondingly increased as the alcohol content of theinjected fuel increases.
 16. A method for an engine, comprising:selectively deactivating one or more engine cylinders; during a firstreactivation, operating reactivated cylinders with fuel injected as asingle injection; and during a second reactivation, operating thereactivated cylinders with fuel split injected into an intake strokeinjection and a compression stroke injection.
 17. The method of claim16, wherein the second reactivation occurs after a longer duration ofdeactivation while the first reactivation occurs after a shorterduration of deactivation.
 18. The method of claim 17, furthercomprising, during the first reactivation, maintaining spark timing ofthe reactivated cylinders, and during the second reactivation, retardingspark timing of the reactivated cylinders.